Did you run the script through SSH terminal? If yes, then just a tiny hiccup anywhere from your computer to the server, might reset the secure tunnel.
If you want to run something all night long, then you’re better of by letting the server handle it - if that’s not possible, because how the script is created, then you can run it through the Console, which you can access through the DigitalOcean control panel.

Hello,

Are you trying to SSH using your SSH key or just with a password?

Note that by default on most Linux distributions password authentication is disabled.

If you don’t have your SSH key on file, you could enable password authentication by following the steps here:

https://www.digitalocean.com/docs/droplets/resources/lost-ssh-key/

After that, you could use PuTTY and authenticate with a password.

Hope that this helps!
Regards,
Alex